This disclosure relates to releasable attachment devices of the type used to fasten, retain, or latch together components of an apparatus or a structure that are to be separated or released under controlled conditions.
Hook and loop type separable fasteners are well known and are used to join two members detachably to each other. These types of fasteners generally have two components disposed on opposing member surfaces. One component typically includes a plurality of resilient hooks while the other component typically includes a plurality of loops. When the two components are pressed together they interlock to form a releasable engagement. A variation on the hook and loop motif is the hook and hook motif. The resulting joint created by the engagement is relatively resistant to shear and pull forces, and weak in peel strength forces. As such, peeling one component from the other component can be used to separate the components with a minimal applied force. As used herein, the term “shear” refers to an action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relatively to each other in a direction parallel to their plane of contact. The term “pull force” refers to an action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to move relative to each other in a direction perpendicular to their plane of contact.
Magnetorheological (MR) fluids and elastomers are known as “smart” materials whose rheological properties can rapidly change upon application of a magnetic field. MR fluids are suspensions of micrometer-sized, magnetically polarizable particles in oil or other liquids. As shown in FIG. 1, when a MR fluid 2 is exposed to a magnetic field, the normally randomly oriented particles 4 within the fluid 6 form chains of particles in the direction of the magnetic field lines. The particle chains increase the apparent viscosity (flow resistance) of the fluid. MR elastomers are suspensions of micrometer-sized, magnetically polarizable particles in a thermoset elastic polymer or rubber. The stiffness of the elastomer structure is accomplished by changing the shear and compression/tension moduli by varying the strength of the applied magnetic field. The MR fluids and elastomers typically develop structure when exposed to a magnetic field in as little as a few milliseconds. Discontinuing the exposure of the MR fluid or elastomers to the magnetic field reverses the process and the fluid returns to a lower viscosity state or the elastomer returns to its lower modulus state. MR fluids enclosed in structural elements have been disclosed in U.S. Pat. No. 5,547,049.